dt-bindings:iio:adc: convert adc.txt to yaml

Each driver that uses this will need to use a $ref
We can't always enable it like most of the generic bindings due to
channel@X matching far more widely than IIO.
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20201031182423.742798-2-jic23@kernel.org

Documentation/devicetree/bindings/iio/adc/adc.txt

-Common ADCs properties
-
-Optional properties for child nodes:
-- bipolar : Boolean, if set the channel is used in bipolar mode.
-- diff-channels : Differential channels muxed for this ADC. The first value
-		specifies the positive input pin, the second value the negative
-		input pin.
-- reg: The channel number.
-- label: Unique name to identify which channel this is.
-
-Example:
-	adc@0 {
-		compatible = "some,adc";
-		...
-		channel@0 {
-			reg = <0>;
-			label = "channel_0_name";
-			bipolar;
-			diff-channels = <0 1>;
-			...
-		};
-
-		channel@1 {
-			reg = <1>;
-			label = "channel_1_name";
-			diff-channels = <2 3>;
-			...
-		};
-	};

Documentation/devicetree/bindings/iio/adc/adc.yaml

+# SPDX-License-Identifier: GPL-2.0
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/iio/adc/adc.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Generic IIO bindings for ADC channels
+
+maintainers:
+  - Jonathan Cameron <jic23@kernel.org>
+
+description:
+  A few properties are defined in a common way ADC channels.
+
+properties:
+  $nodename:
+    pattern: "^channel(@[0-9a-f]+)?$"
+    description:
+      A channel index should match reg.
+
+  reg:
+    maxItems: 1
+
+  label:
+    $ref: /schemas/types.yaml#/definitions/string
+    description: Unique name to identify which channel this is.
+
+  bipolar:
+    $ref: /schemas/types.yaml#/definitions/flag
+    description: If provided, the channel is to be used in bipolar mode.
+
+  diff-channels:
+    $ref: /schemas/types.yaml#/definitions/uint32-array
+    maxItems: 2
+    minItems: 2
+    description:
+      Many ADCs have dual Muxes to allow different input pins to be routed
+      to both the positive and negative inputs of a differential ADC.
+      The first value specifies the positive input pin, the second
+      specifies the negative input pin.
+
+additionalProperties: true